Many computer systems include both disk storage drives and tape drives. Commonly the disk storage drive temporarily stores data and programs which are periodically retrieved and restored while the central processing unit (CPU) is executing programs. Tape drives are more commonly used to store data and programs on a longer term basis than the disk storage. It is also quite common to transfer data from a disk storage drive to a tape drive and vice versa. The two I/O devices, however, generally have different operational characteristics. A disk storage drive is a random access device where data is recorded on concentric tracks having a particular format, normally divided into sectors. A read/write transducer is selectively positioned to the track on which data is to be read or written. A particular sector of the track is then located by the transducer as the disk rotates. The access time is relatively fast compared to access time of a tape drive which reads and writes data as tape moves from a supply source to a tape takeup, usually in the form of reels. Tape drives, however, can operate in a start/stop mode or in a streaming mode. When operating in the start/stop mode, the tape drive must be able to stop the tape in the interblock gap as data is read or written in blocks. For a low cost, medium performance tape drive, because of the high inertia of the reels, the tape must be driven at less than normal speed in order to stop the tape in the interblock gap. If the tape isn't stopped in the interblock gap then the tape has to be repositioned and this is very time consuming.
An alternative is to operate the tape drive in the streaming mode, but in the past the I/O device control subsystem had to be dedicated to the operation so as to assure that the next command for the drive be received within a predetermined time after completion of the previous command. Therefore, the disk storage drive had to be available when needed by the tape drive. The disk storage drives, however, may contain data or have space to write data in addition to data related to the tape operation. Consequently, if the disk storage drive is dedicated or locked to the tape operation, total system performance is degraded. The CPU, for example, could involve the disk storage drive during the interblock gap time for some other job, if there were assurance that the disk storage drive would complete this other command prior to the next command for the tape job. If it didn't then the time consuming operation of repositioning the tape would be required.
The present invention allows the tape to operate in the streaming mode without performance degradation. This is accomplished by operating the I/O control subsystem in a non dedicated mode.